M1: constraint chassis (springs, servo steering, motor wheels)

- New shared Chassis.luau: geometry constants + per-car tuning from stats
- CarManager.buildCarModel rebuilt: prismatic+spring suspension per corner,
  servo-hinge steering on front knuckles, motor-hinge driven wheels,
  hidden dense ballast for anti-roll; VehicleSeat kept as input device only
- New client ChassisController: drives motors/servos from seat
  ThrottleFloat/SteerFloat per Heartbeat; speed-sensitive steering lock;
  engine braking on throttle release
- Driver gets network ownership of every unanchored part (constraint
  assemblies are separate; missing one causes stuttery physics)
- Docs: MESH_SWAP.md rewritten for shell-on-chassis workflow; ROADMAP updated

docs/ROADMAP.md

@@ -14,12 +14,14 @@ Spawn on the neon expressway, drive a car, switch cars, respawn. RX-7 demo-locke
 to owner + son. Cars persist between sessions. CI green on a self-hosted runner.
 
 ## M1 — Make driving feel GOOD (the real work, do this next)
-- [ ] Import the **real RX-7 mesh**; swap `CarManager.buildCarModel` only.
-      See `docs/MESH_SWAP.md` for the exact steps.
-- [ ] Adopt a proven **constraint chassis** (A-Chassis or a modern equivalent)
-      for real suspension/grip instead of the raw VehicleSeat. Do NOT hand-roll
-      physics.
-- [ ] Tune acceleration curve, grip, and the chase camera.
+- [x] Adopt a **constraint chassis** for real suspension/grip instead of the raw
+      VehicleSeat: springs + prismatic suspension, servo steering (speed-sensitive
+      lock), motor-driven wheels, engine braking, ballast anti-roll. Shared tuning
+      in `src/shared/Chassis.luau`; client-side `ChassisController` drives it
+      (driver owns network physics). **Needs a Studio playtest to tune feel.**
+- [ ] Import the **real RX-7 mesh**; weld the shell onto the chassis `Body`.
+      See `docs/MESH_SWAP.md` for the exact steps (needs Studio for asset upload).
+- [ ] Tune acceleration curve, grip, and the chase camera (after first playtest).
 - [ ] Sound: engine loop, tire screech, boost.
 **Exit test:** the RX-7 is genuinely fun to just drive around. Ship nothing else
 until this is true.